One more update, if it isn't getting too tedious ;-)
I checked the disabled bundles: |---> defaults read com.macromates.textmate OakBundleManagerDeletedBundles
The domain/default pair of (com.macromates.textmate, OakBundleManagerDeletedBundles) does not exist
Which seemed odd. So I opened the prefs file in Property List editor, and found the array of disabled bundles. I deleted the elements of the array and restarted Textmate. Still no Textmate bundle. :-%
The next step will have to be a completely fresh installation, I think. I hope I don't lose my syntax colouring. I'll let you know how I get on.
Bruno